1. Field of the Invention
The present invention relates to improved electrodes for rechargeable electrochemical cells and, more particularly, to improved electrodes for nickel metal hydride cells. Specifically, the present invention relates to an improved electrode design for nickel metal hydride cells which significantly inhibits the infantile shorting of the wound cells.
2. Description of the Prior Art
Generally, the manufacture and use of electrodes for electrochemical cells, including nickel metal hydride cells, is known to those skilled in the art. Such electrodes usually include a porous or perforated conductive metal substrate onto which is deposited a layer of electrochemically active material. The electrochemically active material may be applied to the substrate as a pressed or pasted material wherein an organic binder provides mechanical integrity. The pasted substrate is then subjected to mild heat and pressure in order to form a compact layer of active material on the substrate. Alternatively, both positive and negative electrodes may be of a sintered design. For example, a perforated or wire mesh steel substrate may be sintered with a carbonyl nickel powder layer or layers to form a porous electrode plate. The resultant porous plate is then conventionally impregnated with a solution of the electrochemically active material. Such techniques of electrode formation are well known in the art.
Once the electrodes are formed, they must then be manufactured into a cell. In one method of manufacturing cylindrical cells, a negative electrode plate and a positive electrode plate are alternated with two pieces of separator material, such as nylon. This assembly is then wound into a spiral such that the surfaces of the positive and negative electrode plates are juxtaposed throughout the cell. These cell components are generally wound around a removal arbor which is positioned within a nest having a diameter approximately that of that of the container of the electrochemical cell. U.S. Pat. No. 4,814,242, U.S. Pat. No. 4,765,799 and U.S. Pat. No. 4,929,519 all disclose Ni--Cd electrochemical cells using latex binder materials of some sort to bind the active material together. However, some are directed to Ni metal hydride cells nor the problem discussed below.
One of the difficulties with the development of the nickel metal hydride cells, including the AB2 and AB5 alloys, is that they have had a tendency to be somewhat brittle and inflexible. Thus, when the electrodes are bent when being wound, there is a tendency for the electrochemically active material to spall and flake. This tendency can at times cause penetration of the separator material and shorting of the cell. This is particularly true during the initial or infantile start-up use of the cells, with up to 10% of cells frequently being lost through internal shorting. Thus, there remains a need for developing nickel metal hydride electrodes, both positive and negative, which are much more flexible, have greater integrity, and do not spall or flake during winding without affecting the cycle life of the cell.